The present invention relates to a process for the preparation of a quartz crucible for use in the preparation of single crystal silicon materials substantially free from crystal void defects. More particularly, the present invention relates to the production of quartz crucibles which contain a very low amount of gases which are insoluble in liquid silicon.
In the production of single silicon crystals grown by the Czochralski method, polycrystalline silicon is first melted down within a quartz crucible. After the polycrystalline silicon has melted and the temperature equilibrated, a seed crystal is dipped into the melt and subsequently extracted while the crucible is rotated to form a single crystal silicon ingot.
High quality quartz glass is an indispensable material in semiconductor technology in producing and processing crucibles because of its purity, temperature stability and chemical resistance. However, because of the direct contact between the very high temperature, aggressive silicon melt and the wall of the quartz crucible, the danger of contamination of the melt with impurities from the crucible is great.
Translucent quartz crucibles are generally produced by a process in which quartz powder is introduced into a mould to be accumulated to form a layer along the inner surface of the mould. The layer of the quartz powder is then heated and fused at the inner surface thereof while the mould is being rotated to produce a quartz crucible having a relatively high bubble content. As used herein, the term "bubbles" refers to bubbles of air or pockets of air contained in the crucible voids. Generally, translucent quartz crucibles contain bubbles ranging from about 50-200 microns in diameter, with the average bubble having a diameter of about 100 microns. There are approximately 70,000 bubbles/cm.sup.3 in translucent crucibles. These quartz crucibles are advantageous in that they have high strength and are relatively easy to form in large sizes. For these reasons, translucent crucibles are widely utilized.
However, translucent crucibles which contain bubbles throughout the entire structure are not without drawbacks. As the silicon melt stays in contact with the crucible during the crystal pulling, which can be up to 100 hours or more for larger diameter ingots or melts of large mass, the melt continuously reacts with and dissolves the crucible. This dissolution of the quartz crucible causes bubbles in the quartz wall to become exposed and burst. This bursting not only releases the gases inside the bubble into the melt, but also can result in the release of quartz particulates into the melt. These particulates in the melt can come into contact with the growing crystal at the melt interface and be incorporated into the crystal structure. When this happens, a resulting structure loss in the crystal can occur which may lead to a decreased throughput.
In order to improve throughput and reduce resulting crystal contamination, crucibles having a reduced size and density of bubbles or, "bubble free zones", on the inside wall portion of the crucible have been utilized. Uchikawa et al. in U.S. Pat. No. 4,956,208 teach producing a quartz glass crucible having a translucent outer layer with a high bubble content and an inner transparent glass layer which is substantially free from bubbles. The transparent, or substantially bubble free, layer is about 0.3 to 3 millimeters thick, and contains a small number of bubbles which have a diameter less than 50 microns. However, such "bubble free zone" crucibles have had limited success in eliminating all crucible related crystal defects. Due to the long duration of crystal pulling required to produce large diameter crystals, i.e. a 100 hours or more, the substantially bubble free zone of Uchikawa et al. may be dissolved before the crystal is complete and the translucent portion of the crucible may then release bubbles into the melt. Alternatively, even if the transparent layer is sufficiently thick, this layer still contains bubbles which are released into the melt and may directly contact the growing crystal. A true "bubble free zone" containing no bubbles has not yet been commercially produced.
Furthermore, it has now been discovered that the bubbles, which are comprised of air in conventionally prepared crucibles, that are released into the melt contain gases which are thermodynamically stable and highly insoluble in liquid silicon. Such unreactive gases, especially argon, can become trapped at the growth interface and cause crystal void defects, or pockets of gas on the crystal surface, which are detected as Large Light Point Defects (LLPDs). Such defects effect up to 4% of wafers sliced from grown crystals and cause these slices to be unfit for grade 1 wafer product.
Therefore, a need still exists in the semiconductor industry for a crucible which will not release a large number of bubbles containing insoluble gases into the polycrystalline melt that subsequently may lead to the formation of gas voids in the resulting crystal.